1. Field of the Invention
This invention relates to an output filter for an oversampling digital-to-analog (hereinafter referred to as D-A) converter, and more particularly to an output filter for an oversampling D-A converter for converting a delta-sigma modulation output obtained by single-bit quantization by D-A conversion and filtering the same.
2. Description of the Related Art
An oversampling D-A converter based on the delta-sigma modulation technique allows reduction of the bit number to 1 and, even where the converter has a low resolution, allows achievement of a high signal-to-noise ratio (S/N ratio) and reduction of distortion caused by D-A conversion. Therefore, oversampling D-A converters are employed in the digital audio field in place of D-A converters of high accuracy which have conventionally been required.
FIG. 1 shows the basic construction of the oversampling D-A converter described above. The oversampling D-A converter shown is constituted from an inner digital filter 1, a delta-sigma modulator 2, a one-bit D-A converter 3, and a post filter 4 which serves as an output filter. An input signal such as a digitalized audio signal is inputted to the delta-sigma modulator 2 after the sampling frequency is raised to from several tens to approximately 200 times the sampling frequency of the input signal by the inner digital filter 1. The digital signal sampled with the raised frequency is quantized by one-bit quantization by the delta-sigma modulator 2 and is then converted into an analog signal by the next one-bit D-A converter 3, whereafter it is sent to the output filter 4 at the next stage. The output filter 4 removes high frequency noise components from the analog signal inputted thereto and outputs a signal of good reconstructed waveform.
The output filter 4 is constituted from an active filter including a plurality of stages, for example, as shown in FIG. 2. Referring to FIG. 2, the one-bit quantization output (complementary outputs) of the delta-sigma modulator 2 are inputted directly to the active filter 4. The present construction can be considered, as far as the input to the active filter 4 is concerned, equivalent to a construction wherein an output analog signal of the one-bit D-A converter 3 of FIG. 1 is inputted to the active filter 4. In particular, since the one-bit quantization output of the delta-sigma modulator 2 is either "1" or "0" and the D-A converter 3 which converts the one-bit output into an analog signal converts the outputs "1" and "0" into signals of amplitudes of, for example, +3 volts and 0 volts, respectively, the one-bit quantization output and the output analog signal of the D-A converter 3 are different only in amplitude and, as an input to the active filter 4, can be considered equivalent.
Digital FIR (Finite Impulse Response) filters, which are widely employed as filters of a constant group delay in the digital technical field, are sometimes constructed in a manner such as shown in FIG. 3 for use as an output filter 4. The FIR filter of FIG. 3 is constructed so as to receive a digital signal of a plurality of bits as an input signal thereto.
The conventional filter described above, however, has the following problems.
When it is intended to apply the conventional FIR filter to the D-A converter of FIG. 1, if D-A conversion is performed on the input side, a large number of analog delay elements are required. An example is shown in FIG. 3 wherein a CCD (Charge Coupled Device) is employed for delay elements D. However, since the CCD does not have a satisfactory characteristic as a delay element and cannot have a high S/N ratio, it cannot be applied to a D-A converter of high accuracy. Further, since a filter which employs a CCD requires a high voltage, it cannot be formed as an integrated circuit together with a large digital circuit.
Regarding the technology in which the active filter of FIG. 2 is employed, in order to construct a high-performance delta-sigma modulator, it is necessary to make the degree of noise shaping filters in the modulator equal to or greater than 4. Accordingly, in order to remove quantization noise that has undergone noise shaping, the cutoff characteristic for high frequencies of the active filter of the output filter 4 at the output stage of the one-bit D-A converter 3 must be somewhat steep. Accordingly, the degree of the active filter 4 must be made equal to or higher than 5.
Since a signal obtained by D-A conversion from one-bit data by the delta-sigma modulator 2 has much higher noise of high frequencies outside the signal bandwidth than another signal obtained by D-A conversion of an ordinary signal of, for example, 16 bits, the frequency characteristic of operational amplifiers employed for the active filter must be wide.
The presence of high-frequency noise outside the signal bandwidth signifies that original signal components in a one-bit D-A conversion output are reduced to a comparatively low level, and accordingly, the filter must process a voltage amplitude several times higher than the voltage amplitude which is originally required for an output.
Furthermore, because an active filter of a single stage cannot remove noise, a filter having a plurality of stages is necessary, thereby resulting in a complicated circuit and a necessity to design the output amplitude of the filter at the first stage with a sufficient margin for the amplitude of an original signal. Accordingly, the output signal amplitude of the filter at the first stage is restricted by the range of the output voltage of the operational amplifier and must be set considerably lower than the power source voltage.
Since a one-bit output includes a large volume of high-frequency noise, the first stage of the active filter must have a frequency characteristic capable of follow-up to extremely high frequencies, resulting in high power consumption. Further, since the ratio between the peak value of the one-bit output and the peak value of an original signal is high, the first stage of the filter must have the characteristics of very low noise and a wide dynamic range. Accordingly, designing the filter to operate with a reduced voltage results in a reduction of the power source voltage and a consequent decrease in the one-bit output voltage. The signal component included in the one-bit output voltage is therefore further reduced, causing the influence of circuit noise to increase, and as a result, the characteristics cannot be assured.
Furthermore, since the cutoff frequency of an ordinary active filter depends upon the product of the capacitance and the resistance value of elements, each of which have dispersion independently of each other, the accuracy of the filter characteristic cannot be maintained due to the dispersion, and it is very difficult to integrate the filter with D-A converters.
An increase of the degree of filters signifies an increase in the number of necessary operational amplifiers, and the increased distortion and noise produced by the operational amplifiers prevents the maintenance of a good S/N ratio.
As a countermeasure for the drawbacks described above, a technique which employs a digital filter such as shown in FIG. 4 for the output filter 4 for one-bit data of the delta-sigma modulator 2 has been proposed by D. K. Su et al. (David K. Su, Bruce A. Wooley, "A CMOS Oversampling D/A Converter with a Current-Mode Semi-Digital Reconstruction Filter," 1993 IEEE International Solid-State Circuit Conference, Digest of Technology Papers, pp. 230-231).
The digital FIR filter of FIG. 4 controls current outputs of MOS transistors of current sources weighted with FIR filter coefficients in accordance with data delayed by delay elements D to produce a current sum of the current outputs and convert the current sum into a voltage by means of a current-to-voltage conversion circuit 5, and is suitable for removing the above-described high-frequency noise.
However, in order to increase the current sources in proportion to the filter coefficients with a given degree of accuracy, the filter must occupy a large area since the threshold voltage levels of the MOS transistors constituting the constant-current sources have a high dispersion. Further, from the point of view of time, about half of the current flows to the ground and is consumed to no purpose, resulting in a high current consumption.
Furthermore, in order to assure good constant-current characteristics of the current sources, MOS transistors must be arranged in cascade connection in two stages as shown in FIG. 4, and accordingly, to obtain a portable device, the power source voltage must be limited to approximately 3 volts.